Crowds have been gathering at the site of a bizarre hole, which suddenly opened up in Lake Berryessa, California last year.

Dubbed the 'portal to hell', the gigantic hole reappeared in the US lake for the first time in 11 years after heavy rains, sparking widespread amazement and attracting a bevy of visitors to Napa County.

But despite all the recent buzz around the scary-looking phenomenon, there's a simple reason for its sudden manifestation.

The hole is actually a man-made spillway known more commonly as the 'Glory Hole', built near Monticello Dam.

Photo: The Glory Hole has been dubbed the 'portal to hell' by amazed onlookers. iStock.

Measuring in at around 22 metres in diameter at its widest point, with a sheer drop of 61 metres, it's not the sort of thing you want to be swimming near.

When the lake reaches capacity, the spillway funnels the excess water out through the reservoir, creating this epic vacuum effect – think of it like an oversized bath plug.

After heavy rains in 2017, the spillway started flowing again for the first time since 2006, attracting a flurry of attention from onlookers.

Photo: With a diameter of 22 metres at its widest point, the water funnel is quite the spectacle. iStock.

Renewed fascination around the gaping hole has seen mesmerising 2017 drone footage by a YouTube user enjoying a second round in the spotlight.

David B almost lost his drone after venturing precariously close to the powerful spectacle, claiming that the water draining into the spillway "created a sort of vacuum that made the drone unstable".

It's footage like this that has catapulted the site into a tourist attraction in recent times, with visitors flocking to catch a glimpse of the mysterious water funnel.

But while the spillway isn't a gateway to hell, it does have a dark past – it's well known that in 1997 a woman swimming in the reservoir was killed after being sucked inside.

Emily Schwalek, 41, was spotted swimming towards the hole and spent a horrifying 20 minutes grasping onto the edge before disappearing down into the tunnel, according to reports.

Not only that, but Lake Berryessa itself is the site of one of America's most infamous and grisly murders – in 1969 the Zodiac killer slayed two students, Bryan Hartnell and Cecelia Shepard, as they enjoyed a picnic near the water.